THIS INVENTION relates to a method of determining the direction of arrival of signals impinging on a plurality of antennae.
Direction finders are devices that determine the direction of arrival of signals impinging thereupon. Direction finders play an important role in spectrum monitoring, as well as in reconnaissance and surveillance applications, as the direction in which transmitters in the vicinity of a direction finder lie can be established. Typically, direction finders comprise an array of antennae which detect signals impinging thereon, with phase and amplitude information from the antennae being correlated to calculate the direction of arrival of the impinging signals.
In situations where strong multipath interference or co-channel signals exist, it is necessary to employ high resolution directions finding algorithms to interpret the data gathered by a direction finder. In conventional direction finders using such high resolution direction finding algorithms, one receiver is provided for each antenna. Since the signal output by each of the antennae must be coherently sampled, multi-channel receivers are required for this task. However, such receivers are relatively expensive and complicated in design.
If one receiver is provided for each antenna, the information received by each of the antennae may be continuously sampled. However, there are disadvantages to this mode of construction. It is costly to provide a large number of receivers, along with associated wiring and so forth, and in applications where the volume and/or weight of the direction finder are critical, the provision of a large number of receivers can be disadvantageous. In addition, the likelihood of equipment failure in the case of a direction finder having a large number of receivers is increased, and maintenance costs are inevitably higher. The Application of high resolution direction finding algorithms requires the receivers to be matched in their phase and amplitude response. To ensure that this situation exists, receiver calibration circuitry and signal sources are required. Hence, it will be appreciated that the need to maintain the calibration of the receivers will increase the cost and complexity of the direction finder.
In view of the above drawbacks, it has been proposed to construct a direction finder having fewer receivers than antennae. It will be appreciated that a consequence of this arrangement is that the information received by each of the antennae can no longer be continuously sampled simultaneously, and various techniques have been developed to compensate for this. For instance, it has been proposed to provide a time-varying pre-processing network between the antennae and the reduced number of receivers, and to implement an appropriate algorithm to interpret the data output from the time-varying pre-processing network. However, the proposed method requires at least two receivers, and shares the same receiver calibration and coherency requirements as conventional high resolution direction finding algorithms.
A further technique employs an array of antennae and a single receiver. The antennae in the array may be connected individually to an output by a switching network. In use of such an arrangement, the information output by a single antenna is received at the receiver at any one time, and knowledge of the geometric layout of the antennae comprising the array is employed to establish, from the data output by the receiver, the direction of arrival of signals impinging on the array. It will be understood that the above-mentioned drawback regarding calibration is overcome when a single receiver is used.
U.S. Pat. No. 5,497,161 describes a method of achieving high-resolution direction finding with a single receiver that sequentially samples the antennae in an antenna array. In use of this method, a vector of antenna array outputs that might have been gathered from each antenna simultaneously is reconstructed, by compensating for the time-delays arising from the sequential sampling using delay lines. Alternatively, the time-delays can be compensated for by means of digital signal processing to realign the received data samples. Once the vector of array outputs has been reconstructed, conventional high-resolution direction finding algorithms can be employed to determine the directions of arrival of signals impinging on the antenna array.
The drawback of this approach (when either delay lines or digital processing are employed) is that the antenna analogue to digital conversion sampling rate must be larger than 2N times the receiver bandwidth, where N is the number of antennae. This will result in implementation difficulties in modern spectral monitoring and surveillance applications, where the instantaneous bandwidth of the receiver is typically large. For example, a typical 5 element antenna array direction finder with a 10 MHz instantaneous bandwidth will require a sampling rate greater than 100 MHz (5xc3x9710xc3x972 MHz). Hence, when employing this method in wide instantaneous receiver bandwidth applications, very high-speed radio frequency switches and analogue digital conversion will be required, even though the bandwidth of the received signal may be much smaller than the receiver bandwidth. However, the need for higher speed devices and the corresponding increase in the complexity of the circuitry will increase the cost of the system This increased cost offsets the savings offered by the reduction in the number of receivers.
When delay lines are used to introduce time delays to realign the data received from the antenna array, the lengths of the delay line need to be highly accurate, as errors in the realignment of the data will introduce phase and amplitude distortions to the signal, thereby reducing the signal to noise ratio. To produce delay lines with sufficiently precise lengths is a difficult and expensive process. In addition, the delays introduced by the delay lines will vary with the ambient temperature and humidity. While, in principle, these variations can be compensated for via calibration during power up and further periodic calibration during operations, the need for calibration circuitry will eliminate the advantages of using a single channel receiver. Furthermore, as the lengths of the delay lines are determined according to a particular sampling rate, any changes in the sampling rate will require a set of delay lines having different lengths, leading to a system that is inflexible and difficult to adapt to new circumstances.
Digital signal processing using fast Fourier transforms (FFT""s) to compensate for the time delays between samples received from antennae in an array has also been proposed. The FFT processing attempts to finely channelise the signal to a sufficiently small bandwidth that the time delays can be approximated by phase shift. Reliable approximation can only be achieved when the largest time delay is very much smaller than the inverse of the bandwidth. However, very high antenna and analogue to digital conversion sampling rates and long integration times are required to achieve the desired channelised bandwidth. Nevertheless, as a result of this approximation, the reconstructed array output will suffer a reduced signal to noise ratio due to signal decorrelation. In addition, this approach will require N FFT processors to align the data output by the array, which will further increase the cost and complexity of the direction finder.
It is an object of the present invention to seek to provide a method of determining the angle of arrival of the signal, which method alleviates some or all of the above drawbacks.
Accordingly, the present invention provides a method of determining the angles of arrival of a plurality of signals, the method comprising the steps of: providing a plurality of antennae, sampling, in turn, the output from each of the antennae, to obtain a set of samples, each of the samples relating to different points in time; performing a Fourier transform on the set of samples, thereby producing a sequence of spectral lines; identifying each of a plurality of signals impinging on the antennae from the configuration of the spectral lines; and calculating, from phase and amplitude data gathered from at least respective subset of the spectral lines, the directions of arrival of each of the plurality of signals at the antennae.
Advantageously, the plurality of antennae are spatially separated from one another.
Alternatively, the plurality of antennae are provided at substantially the same location as one another.
Preferably, the plurality of antennae have differing gain characteristics from one another.
Conveniently, each of the subsets of the spectral lines relates to one of the plurality of signals.
Advantageously, the method further comprises the formulation of a vector comprising one of the subsets of the spectral lines.
Preferably, the step of identifying the plurality of signals impinging on the antennae from the configuration of the spectral lines comprises the step of identifying the signals impinging on the antennae from the positions of the spectral lines with respect to one another.
Conveniently, the method further comprises the step of determining the frequency of each of the plurality of signals.
Advantageously, the step of determining the frequency of each of the plurality of signals comprises the step of determining the frequency of each signal from the positions of the spectral lines relating to that signal.
Preferably, the outputs from the antennae are sampled by a single receiver.
Another aspect of the present invention provides a direction finder comprising: plurality of antennae; receiver, operable to collect samples from the antennae; switching network, operable to connect the antennae, in turn, to the receiver; and a processor operable to: receive a set of samples from the receiver, each of the samples relating to different points in time; perform a Fourier transform on the set of samples, thereby producing a sequence of spectral lines; identify a plurality of signals impinging on the antennae from the configuration of the spectral lines; and calculate, from phase and amplitude data gathered from at least a subset of the spectral lines, the direction of arrival of each of the plurality of signals at the antennae.
Conveniently, the plurality of antennae are spatially separated from one another.
Alternatively, the plurality of antennae are provided in substantially the same location as one another.
Advantageously, the plurality of antennae have differing gain characteristics from one another.
A further aspect of the present invention provides a direction finder comprising: a plurality of antennae; a receiver, operable to collect samples from the antennae; a switching network, operable to connect the antennae, in turn, to the receiver; and processing means operable to: receive a set of samples from the receiver, each of the samples relating to different points in time; perform a Fourier transform on the set of samples, thereby producing a sequence of spectral lines; identify a plurality of signals impinging on the antennae from the configuration of the spectral lines; and calculate, from phase and amplitude data gathered from at least a subset of the spectral lines, the direction of arrival of each of the plurality of signals at the antennae.